A pilot field study in the eastern tropical Pacific was conducted as part of a larger, cooperative effort to investigate air-sea coupling in the tropical regions of the Americas and its links to climate variability in the Americas. The observational program began in April 1997 when two surface moorings were deployed at the 3° S (cold tongue) and 10° N (ITCZ) on 125° W. Each mooring carried two complete sets of meteorological sensors (wind velocity, air and sea temperature, incoming short-wave radiation and incoming long-wave radiation, humidity, barometric pressure, precipitation, surface currents) using redundancy to ensure that a complete time series of all variables will be collected, thus permitting calculation of the heat, mass, and momentum fluxes via the bulk formulae. The moorings also carried oceanographic sensors (temperature, conductivity and current) placed in the top ~150m of the ocean to monitor the upper ocean variability. The two moorings were recovered in September 1998. This project covers the scientific analysis and publication of the mooring data that were collected. The focus of this study will be on sea surface temperature, air-sea fluxes, air-sea coupling, and upper ocean variability in the PACS study region.

Our specific analysis objectives are to provide:

• Accurate quantification of the air-sea fluxes of heat, fresh water and momentum in the ITCZ/Cold Tongue Complex (CTIC) region on times scales from hourly to annual.

• Description of the temporal evolution of the upper ocean at two contrasting sites, cold tongue and ITCZ, and quantification of the relative roles of ocean processes and air-sea interaction in creating SST anomalies.

• Analysis of a well-sampled meridional section, 125°W, during a strong El Nino event.

Methodology:
Our analysis will examine the local interaction between the atmosphere and ocean in the eastern tropical Pacific during the 1997-1998 warm event and during the onset of the ongoing cold event using data from two air-sea interaction surface moorings and three cruises in the region. This is the first time the complete characterization of the air-sea exchange of heat, moisture and momentum has been obtained for extended periods of time at remote locations in the eastern tropical Pacific. Accurate fluxes of momentum, heat, and freshwater will be determined using techniques developed in TOGA COARE (Tropical Ocean-Global Atmosphere Coupled Ocean Atmosphere Response Experiment). The fact that the 18 month mooring record coincides perfectly with the one of the largest ENSO warm events on record makes this data even more valuable. Extensive collaborations with scientists from NCEP, ECMWF, NCAR, NASA, SUNY, U. Miami, U. Washington and other institutions will extend the scope of the analysis.

We will initially focus on the analysis of upper ocean and air-sea flux variability and evolution in the two regimes sampled by the moorings and contrast the air-sea interaction at the two sites which at times spanned the meridional gradients in SST associated with the cold tongue/ITCZ complex and the onset of La Niña conditions. We will also explore the air-sea fluxes in relation to convective variability and large-scale atmospheric circulation during the 18 observing period. Variability on time scales from instantaneous (60 sec) to diurnal to seasonal to interannual would be explored. Associated with this effort is the validation of the moored data by intercomparison of redundant systems, evaluation of pre and post deployment calibrations and ship-buoy intercomparison. This will assure that we are working with a data set of the highest quality.

Results and Accomplishments:
We are only just starting analysis work and will update this section as the project continues. We have just completed data processing, quality control and documentation of all the moored data during this project. A data report with the details of this process is completed and will soon be available as a WHOI Technical Report as will a CDROM containing the time series of air-sea fluxes at the two sites. We have begun evaluation of gridded products from ECMWF, NCEP, SSM/I and NSCAT.

Future Work:
We plan to analyze the ocean response and the processes that govern the evolution of the upper ocean temperature, salinity and velocity at each site and explore what extent is the local upper ocean heat budget is dominated by surface heat fluxes, horizontal advection, upwelling and entrainment. We seek to extend of our understanding of the air-sea fluxes from the buoy sites to a wider region in a cooperative effort with colleagues at NCEP, NASA, ECMWF and Southhampton Oceanography Centre (UK). We will use the highly accurate buoy fluxes to evaluate and contrast gridded surface flux products derived from in situ, remotely sensed and model fields. By completing the buoy-gridded product intercomparison, we hope to identify systematic errors and improve the fields for regional numerical studies. This would yield a firm foundation and data set for testing tropical ocean models and validating forecast model results. Clearly this is a region and time when remote forcing led to dramatic changes in the thermocline depth and horizontal advection of water masses. The moored data record will allow us to quantify the role of a number of physical processes at those two sites. Then, together with PACS ocean modelers, we plan to explore in three dimensions the physical processes that governed that large scale circulations and hydrographic variability in the eastern tropical Pacific during 1997 and 1998.

Publications resulting from this research:
Anderson, S. P. and R. A. Weller, 1999: Air-sea interaction in the eastern tropical Pacific: An experiment of the Pan American Climate Study (PACS) during the 1997-1998 ENSO. Proceedings of the 23^rd Conference on hurricanes and tropical meteorology. 10-15 January 1999, Dallas, TX. 143-144.

Way, B., W. Ostrom, R. Weller, J. Ware, R. Trask, R. Cole, and J. Donovan, 1998: Pan American Climate Study (PACS) Mooring Deployment Cruise Report, R/V Roger Revelle Cruise Number 4, 9 April – 5 May 1997. WHOI Technical Report WHOI-98-07, pp. 71.

Trask, R., R. Weller, W. Ostrom, B. Way, 1998: Pan American Climate Study (PACS) Mooring Deployment Cruise Report, R/V Thomas Thompson Cruise Number 73, 28 November to 26 December 1997. WHOI Technical Report WHOI-98-18, pp. 103.

Ostrom, W., B. Way, S. Anderson, B. Jones, E. Key, G. Yuras, 1999: Pan American Climate Study (PACS) Mooring Deployment Cruise Report, R/V Melville Cruise PACS03MV, 6 September to 30 September 1998. WHOI Technical Report WHOI-99-06, pp. 74.

Anderson, S. P., K. Huang, N. Brink, M. Baumgartner and R. Weller, 2000: Pan American Climate Studies Data Report. WHOI Technical Report in press.

Principal Investigators:

Steven P. Anderson
sanderson@whoi.edu
Phone: (508) 289-2876
Fax: (508) 457-2163

Robert A. Weller
rweller@whoi.edu
Phone: (508) 289-2508
Fax: (508) 457-2163

Institution:
Woods Hole Oceanographic Institution
Woods Hole, Massachusetts 02543

Links:
http://www.whoi.edu

http://uop.whoi.edu

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